For many years, anglers fishing the lower reaches of the River Thames have reported hooking enormous fish, particularly at night. It is a pattern familiar on other rivers, such as the Severn and Trent, and the culprits are almost certainly wels catfish (Silurus glanis).
The wels is a freshwater giant, often growing in excess of three metres in length and rumoured to reach five metres and 275kg in weight in the Volga River system in Ukraine and Russia. Native to central-eastern Europe and parts of Asia, wels were introduced to rivers in western Europe from the mid-20th century for their sporting qualities.
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In some river systems, such as The Ebro in Spain, they have flourished, while the disappearance of many native fish species is unlikely to be a coincidence. Wels are not fussy eaters, using their whiskers to search for food alive or dead. Cavernous mouths and long, slug-like bodies enable them to swallow large items; remains of wild boar and deer have been found in larger specimens.
Wels were first brought to the UK in the late 19th century by Herbrand Russell, who stocked them in a lake at his Woburn Abbey home. More recently, they have been stocked in an estimated 500 lakes nationwide, although this can only be done under licence.
Those uncaught monsters in our river systems have either escaped or more likely been unofficially stocked. Their long-term impact in such places is uncertain, but they are certainly capable of breeding in British river systems and gaining huge size.
More on non-native species:
